Medical hypothesis The Great Leap Forward: the anatomic ... · Medical hypothesis The Great Leap Forward: the anatomic basis for the acquisition of speech and obstructive sleep apnea

Medical hypothesis

The Great Leap Forward: the anatomic basis for the acquisition of speech

and obstructive sleep apnea

Terence M. Davidson*

Department of Otolaryngology – Head and Neck Surgery, University of California, San Diego and the VA San Diego Health Care System, San Diego, CA, USA

Received 19 June 2002; received in revised form 23 October 2002; accepted 30 October 2002

Abstract

Obstructive sleep apnea is an anatomic illness caused by evolutionary changes in the human upper respiratory tract. These changes include

shortening of the maxillary, ethmoid, palatal and mandibular bones, acute oral cavity-skull base angulation, pharyngeal collapse with anterior

migration of the foramen magnum, posterior migration of the tongue into the pharynx, descent of the larynx and shortening of the soft palate

with loss of the epiglottic–soft palate lock-up. While it is commonly believed that some of these changes had positive selection pressures for

bipedalism, binocular vision and locomotion, development of voice, speech and language ultimately became a substantial contributing

factor. Here it is shown that these changes are the anatomic basis of obstructive sleep apnea.

q 2003 Elsevier Science B.V. All rights reserved.

Keywords: Obstructive sleep apnea; Sleep disordered breathing; Upper respiratory tract; Anatomy; Evolution

1. Introduction

With the possible exception of brachycephalic dogs, such

as the English Bulldog, man is the only mammal that

experiences obstructive sleep apnea (OSA) [1,2]. The adult

Homo sapiens supralaryngeal vocal tract (SVT) differs from

that of close ancestors and other mammals by: the presence

of a short face or splanchnocranium made up of the

mandible, palate, ethmoid, maxilla and sphenoid; a narrow

elongated supralaryngeal vocal tract (SVT); an anterior

foramen magnum and oropharyngeal tongue; a descended

larynx and shortened soft palate with loss of the epiglottic–

soft palate lock-up; and an acute oral cavity–skull base

angle (Table 1). These anatomic features facilitated man’s

ability to speak and to develop language (Table 2). This very

same anatomy, a product of man’s evolution, predisposed

man to the development of OSA.

The natural selection pressure for speech and language

was so strong that the undesired consequence of OSA was

carried forward to modern man. Based on this reasoning,

obstructive sleep apnea is an anatomic illness.

As the terminology for this region can be confusing,

Table 3 defines the overlapping nomenclature.

2. Acquisition of speech

Speech and the ability to communicate separated man

from the remainder of the animal kingdom and permitted

humans to evolve into advanced civilization. Jared Dia-

mond, a physiologist at UCLA, has labeled this evolutionary

change ‘The Great Leap Forward’ [3]. Diamond postulates

that The Great Leap Forward occurred approximately

40 000 years ago (40 ka). Prior to that, man possessed

tools and a sizeable brain, but little progress had occurred

for hundreds of thousands of years. Forty ka ago, the SVT

anatomy and the necessary neural connections were

completed so that man could speak and create language.

Diamond sites strong selection pressure for voice, speech

and language. This pressure was a substantial contributing

factor for evolutionary change in the anatomy of the SVT

[3].

One can easily imagine the survival advantage of those

groups that could speak over those that could only grunt.

Speaking humans could communicate messages pertaining

to defense and food acquisition and could learn from the

generations that came before them. Prior to the presence of

1389-9457/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S1389-9457(02)00237-X

Sleep Medicine 4 (2003) 185–194

www.elsevier.com/locate/sleep

* University of California, San Diego, 9500 Gilman Drive 0617, La Jolla,

CA 92093-0617, USA. Tel.: þ1-858-822-4229; fax: þ1-858-534-7672..

E-mail address: [email protected] (T.M. Davidson).

speech, information was acquired slowly and knowledge

was not easily passed on. With speech came language and

then the written word. When language was developed and

then recorded, massive bodies of knowledge were accumu-

lated. Man could now learn from more that his own

experiences. The following excerpts summarize Diamond’s

reasoning [3].

The identity of the ingredient that produced the Great

Leap Forward poses an archaeological puzzle without an

accepted answer. It doesn’t show up in fossil skeletons. It

may have been a change in only 0.1 percent of our DNA.

What tiny change in genes could have had such

enormous consequence?Like some other scientists who

have speculated about this question, I can think of only

one plausible answer: the anatomical basis for spoken

complex language.The answer seems to involve the

structure of the larynx, tongue, and associated muscles

that give us fine control over spoken sounds. Like a Swiss

watch, all of whose many parts have to be well designed

for the watch to keep time at all, our vocal tract depends

on the precise function of many structures and

muscles.That’s why it’s plausible that the missing

ingredients may have been some modifications of the

protohuman vocal tract to give us finer control and permit

formation of a much greater variety of sounds.It’s easy to

appreciate how a tiny change in anatomy resulting in

capacity for speech would produce a huge change in

behavior. With language, it takes only a few seconds to

communicate the message, “Turn sharp right at the fourth

tree and drive the male antelope toward the reddish

boulder, where I’ll hide to spear it.“ Without language,

two protohumans could not brainstorm together about

how to devise a better tool or about what a cave painting

might mean. Without language, even one protohuman

would have had difficulty thinking out for himself or

herself how to devise a better tool.

3. Hypothesis

The anatomic piece of the Great Leap Forward is a 1:1

ratio of the vertical (SVTV) to horizontal (SVTH) portions of

the SVT. SVTH includes the vocal tube from the lips to the

pharynx. SVTV includes the vocal tube from the pharynx to

the vocal cords [4], Additional required changes include

buccal speech and a narrow, distensible, angulated SVT.

The hypothesis is that the anatomic changes that created this

SVT contributed to Homo sapiens’ recent upper respiratory

tract evolution and resulted in the development of

obstructive sleep apnea (OSA).

4. Methods

Comparative anatomy and the principle of ontogeny

recapitulates phylogeny were used to examine the evol-

utionary changes from pre-hominid primates to anatomi-

cally modern (a.m.) Homo sapiens. Pan troglodytes is a.m.

Homo sapiens’ closest living primate relative [3], and is

used for much of this comparison.

5. Klinorynchy

Laurence Barsh describes klinorynchy as the posterior

migration of the splanchnocranium (facial skeleton) under

the neurocranium (brain case) [1]. This has resulted in

several changes in the maxilla, palate, ethmoid and

mandible. According to study of the phylogenetic evolution

of the splanchnocranium, the maxilla and other facial bones

moved posteriorly [5,6]. The mandible followed, moved

posteriorly, and rotated downward, as reflected in an obtuse

gonial angle. Absent other change, this posterior migration

would cause the pharynx to be compressed. In order to

preserve the pharynx for both respiration and deglutition,

the mandible, maxilla, ethmoid and palate were shortened.

Fig. 1 morphs the common chimpanzee to modern man,

showing these changes.

Further evidence of this change is seen in the teeth. The

dentition in man is crowded, as proven by the fact that man

is the only primate with impacted third molars. David

Roberts writes, “It is well known that dentitions are more

Table 1

Anatomic changes facilitating speecha

Short maxilla/mandible

Short ethmoid and palate

Anterior foramen magnum

Acute cranial base angulation

Oropharyngeal tongue

Descended larynx

Shortened soft palate

Loss of epiglottic–soft palate lock-up

Narrow, distensible supralaryngeal vocal tract (SVT)

1:1 ratio of the SVTV to SVTH

a The SVT includes the larynx, pharynx, nasal cavity and oral cavity.

SVTV is the vertical segment and SVTH is the horizontal segment.

Table 2

Anatomic requirements and SVT changes for modern speech

Requirements Changes

1:1 ratio SVTV to SVTH Klinorynchy

Laryngeal descent

Buccal speech Laryngeal descent

Shortened soft palate

Loss of epiglottic–soft palate lock-up

Narrow, distensible,

angulated SVT

Klinorynchy

Anterior migration of foramen magnum

Oropharyngeal tongue

Acute craniobase angulation

T.M. Davidson / Sleep Medicine 4 (2003) 185–194186

conservative in terms of genetic change than are other parts

of the skeleton” [5]. A.E.W. Miles writes, “In summary,

over the past 20 million years or so man’s dentition has been

slower to change than other parts of him” [7]. This is shown

schematically in Fig. 2.

A noteworthy part of this change is that as the dental

arches shorten, they expand laterally. This has obvious

implication for expansive orthodonture to prevent OSA.

These effects, namely foreshortening of the maxilla, palate,

ethmoid and mandible, resulted in the shortening of the oral

cavity (SVTH) and contributed to the narrowing of the

pharynx [4].

6. Laryngeal descent

V.E. Negus describes descent of the larynx in the classic

text, The Comparative Anatomy and Physiology of the

Larynx. Negus reviews comparative anatomy of the larynx

beginning with the earliest creature that ventured from

water onto land for food or burrowed in the mud and was

forced to breathe air during the dry season. He then follows

the progression of the organ. The larynx evolved very early

as a protective sphincter for the air-containing sac that

ultimately became the lungs. Negus views the larynx as an

organ developed primarily to separate the alimentary and

respiratory tracts [8].

Negus describes the evolution of the larynx in relation to

the development of speech, “From the observation of all

species in respect to their anatomical structure and their

physiological necessities, it is concluded that the primary

function of the epiglottis is to subserve the sense of smell”

[8]. As many animals are dependent on their sense of smell

to detect prey and to avoid noxious foods and dangerous

predators, it was mandatory for the olfactory tract to be open

at all times, especially during inspiration, expiration and

deglutition.

The larynx and the epiglottis in all animals reside

superior to the oropharynx. In many mammals, including

dolphins, bears and dogs, the larynx sits at the skull base.

The monkey’s larynx is between the skull base and the first

cervical vertebrae. The cat and the squirrel have the lowest

Fig. 1. Klinorynchy as demonstrated by the evolution from Pan troglodytes

to Homo sapiens. The lower right figure is a midsagittal view of Pan

troglodytes. The upper left figure is a midsagittal view of Homo sapiens,

with the tongue drawn in the awake position, i.e. with the tongue base

pulled forward. The upper right figure shows the splanchnocranium of Pan

troglodytes combined with the neurocranium of Homo sapiens. The lower

left figure shows the neurocranium of Pan troglodytes combined with the

splanchnocranium of Homo sapiens. The key changes have not been driven

by the expansion of the neurocranium over the mid-face, but rather the

retrusion and inferior rotation of modern man’s mid and lower face. Visible

Productions, 2001.

Table 3

Termsa

Upper respiratory tract The air passage above the vocal cords, including nose, nasopharynx, oropharynx and larynx

Upper aerodigestive tract Upper respiratory tract plus the hypopharynx and oral cavity. Includes the nose, oral cavity, nasopharynx,

oropharynx, hypopharynx, and larynx. If one opens the mouth, the oral cavity becomes part of the upper

respiratory tract. With this inclusion, the only difference between the upper respiratory tract and the upper

aerodigestive tract is the hypopharynx

Supralaryngeal vocal cord tract (SVT) The voice passage from vocal cords to oral lips; therefore the supraglottis, oropharynx and oral cavity. The

vertical segment, SVTV, extends from the vocal cords to the top of the oropharynx. The horizontal segment,

SVTH, extends from the lips to the posterior wall of the pharynx

Obstructive sleep apnea (OSA) Disruption in sleep caused by anatomic obstruction in the upper respiratory tract.

Sleep disordered breathing (SDB) A broader category of breathing disorders during sleep, including OSA, snoring, Cheyne Stokes breathing,

hypoventilation syndrome, upper airway resistance syndrome (UARS). SDB and OSA are often used

interchangeably. SDB is increasingly the preferred term among sleep medicine experts

Klinorynchy The migration of the splanchnocranium (face) under the neurocranium [1,5]

Homo sapiens Genus and species of man, anatomically characterized by “a high round cranium, a chin, a small orthognathic

face, as well as reduced masticatory apparatus and brow ridges.” [13]. Anatomically modern Homo sapiens first

appeared 250–300 ka and is designated as a.m. Homo sapiens or as subspecies Homo sapiens sensu stricto

a As different definitions describing the upper respiratory tract have been developed for different purposes, the nomenclature is overlapping and potentially

confusing. These are terms used in this paper.

T.M. Davidson / Sleep Medicine 4 (2003) 185–194 187

lying larynx, which resides at the top of the first cervical

vertebrae.

Only man has a descended larynx. The larynx is located

between the third and fourth cervical vertebrae in the human

newborn and is located at the bottom of the fourth cervical

vertebrae in the human adult, as depicted in Fig. 3. Fig. 4

shows these relationships in the dog, the chimpanzee, the

infant human and the adult human. In terms of the

relationship between soft palate and epiglottis, it is found

that the majority of animals do not have a uvula. Instead, the

soft palate extends posteriorly and inferiorly, further

separating the airway from the alimentary tract. The

uvula, in fact, is the remnant of the long soft palate [8].

Negus’ view of the evolution of speech is summarized as

follows. As primates assumed an upright position, they

began to rely more on vision than olfaction. This permitted

the degeneration of the sense of smell and liberated the soft

palate [8].

The degeneracy of the sense of smell liberated the soft

palate from the necessity of contact with the epiglottis

and allowed a gap to be interposed between the two.

After separation had occurred it became easy for

laryngeal sounds to escape from the mouth and for the

oral cavity and lips to enter into the formation of vowel

sounds and consonants.

The lock-up between the soft palate and epiglottis is seen

throughout the animal kingdom. Fig. 5 shows the epiglot-

tic–soft palate lock-up in the goat, Capra hircus. It is only

in man that this lock-up is lost, due to laryngeal descent and

shortening of the soft palate. These changes allowed man to

acquire buccal speech.

7. Oropharyngeal tongue

Crelin notes that man is the only animal whose tongue

resides partially in the pharynx. In all other animals,

including non-human primates, the tongue resides exclu-

sively in the oral cavity. Crelin does not clarify whether the

Fig. 2. Maxillae of Pan troglodytes, Homo erectus and Homo sapiens. Homo sapiens’ maxilla is short and wide. The teeth are crowded. The shortening of the

maxilla is depicted in the lateral views. The arrows on the figure’s right depict the anterior rim of the foramen magnum and serve as a reference point for the

posterior pharynx. Note the narrowing of the pharynx as depicted by the distance from the posterior maxilla to the anterior foramen magnum. From Miles [7].

Reprinted by permission from The Royal Society of Medicine.

T.M. Davidson / Sleep Medicine 4 (2003) 185–194188

human oropharyngeal tongue is a result of the shortened oral

cavity, descent of the larynx or some other reason [9].

Negus also describes the retro positioning of the tongue.

Negus opines that the tongue is primarily designed for

mastication and that a shorter tongue would do for bolus

formation. As klinorynchy progressed and the jaws receded,

the tongue was pushed posteriorly. The human oral cavity is

far smaller than that of a similar sized non-hominid primate,

yet the tongue remains approximately the same volume. The

tongue is therefore oversized, and according to Negus, has

thus pushed the larynx inferiorly. The tongue now protrudes

into the oropharynx and whereas in most animals the tongue

is relatively flat, the tongue in man is curvilinear, bulky, and

folds both posteriorly and inferiorly [8].

It seems more likely that the larynx descended to enable

speech. The tongue followed the laryngeal descent and filled

the pharynx. The undulating dorsal lingua of an open

mouthed gospel singer is a persuasive example of the

tongue’s role in speech. Perhaps it is not by accident that

humans have an oropharyngeal tongue, but rather by design,

for this organ facilitates both speech and deglutition.

8. Pharyngeal collapse and anterior migration of the

foramen magnum

Crelin also examines the base view of the skull. He notes

that in adult a.m. Homo sapiens the space from the palate to

the foramen magnum is shorter than the same space

belonging to other adult primates. In addition, Crelin writes

that the base of the newborn and young Homo sapiens skull

was similar in proportion to the adult chimpanzee and other

non-hominid primates. The distance and space between the

posterior palate and anterior foramen magnum indicates that

this space was available for the pharynx. For purposes of

olfaction, bigger is better. For purposes of speech, smaller is

better and this is exactly what evolved in humans.

Examination of primate skulls shows that the foramen

magnum is located more anteriorly, the closer one gets to

modern man [9]. While it is opined that this is a favorable

change for man’s upright stance, it can also be argued that

man requires a narrow, distensible pharynx to facilitate

speech.

The anterior migration of the foramen magnum is also

part of the evolutionary change to facilitate speech. This is

seen in Fig. 2.

9. Craniobase angulation

Craniobase angulation is the relationship between the

maxilla, ethmoid, sphenoid and basioccipital bones. This is

the bend in the two-tube SVT, the angulation between SVTV

and SVTH. Lieberman and McCarthy examine the ontogeny

of cranial base angulation in humans and chimpanzees.

They report that craniobase angulation occurred early in

Homo sapiens, and that flexion is seen in humans whereas

extension is found in non-human primates. In a.m. Homo

sapiens, the cranial base flexes 8–168 postnatally, but in

Pan troglodytes (common chimpanzee) 15–288 extensions

are seen. Accepting the premise that ontogeny recapitulates

phylogeny, these changes contribute to the rotation aspects

of klinorynchy. Lieberman postulates that this created an

advantage for the development of speech, for the acute

angle between SVTV and SVTH facilitates speech [10].

10. Speech

Lieberman describes human language from an evol-

utionary perspective in his book Eve Spoke. The production

of speech begins in the larynx. As the vocal cords adduct

and air is expelled, a sound is produced. This is called the

pitch [11].

The major factor that differentiates words in all human

languages, however, is not the pitch of a person’s voice.

The tube above the larynx, called the supralaryngeal

vocal tract (SVT) like the clarinet’s tube, filters the sound

Fig. 3. Epiglottic–soft palate lock-up as viewed from the posterior pharynx.

(a) In the human infant, the epiglottis overlaps the soft palate and food is

diverted laterally around the epiglottis. Alimentation and respiration can

occur concurrently. In animals, there is no uvula and the soft palate hangs

like a curtain, further separating the alimentary and respiratory tracts. (b) In

the human adult, the larynx is descended, the soft palate is shortened and

the epiglottic–soft palate lock-up is lost. While food theoretically channels

around the larynx, there is constant risk of aspiration. As Charles Darwin

wrote, “…every particle of food and drink which we swallow has to pass

over the orifice of the trachea, with some risk of falling into the lungs,

notwithstanding the beautiful contrivance by which the glottis is closed”

[12]. Visible Productions, 2001.

T.M. Davidson / Sleep Medicine 4 (2003) 185–194 189

produced by the larynx.Changing the position of the

pharynx, tongue and lips produces speech. The nasal

cavity plays a minor role in speech production.

The structure of the SVT provides man with the ability to

vocalize the vowels and consonants that constitute human

speech. Lieberman and others point out that the major

disadvantage of the SVT is that food can be accidentally

inhaled. He points out that in 1859 Darwin noted, “…the

strange fact that every particle of food and drink we swallow

has to pass over the orifice of the trachea with some risk of

falling into the lungs” [11,12]. Lieberman writes [11],

The human vocal tract has other liabilities. Our

mouths and jaws are shorter than those of non-

human primates are. If you compare a human jawbone

and upper jaw with a Neanderthal’s it becomes

obvious that there is lots of space for Neanderthal

Fig. 4. The epiglottic–soft palate relationship and the descent of the larynx. (a) In the dog, Canis familiaris, the tongue resides exclusively in the oral cavity, the

epiglottis and soft palate are locked up and the larynx resides high in the neck. (b) In the common chimpanzee, Pan troglodytes, the tongue resides exclusively

in the oral cavity, the epiglottic–soft palate relationship persists and the larynx is high. (c) In the infant Homo sapiens, the epiglottic–soft palate lock-up

persists (ontogeny recapitulates phylogeny), the larynx is high and the tongue is primarily in the oral cavity. As the juvenile matures, the larynx descends and

the tongue falls into the pharynx. (d) In the adult Homo sapiens, the epiglottic–soft palate lock-up is lost. The larynx is descended. The tongue protrudes into

the pharynx. Visible Productions, 2001.

T.M. Davidson / Sleep Medicine 4 (2003) 185–194190

teeth. Neanderthals never had impacted wisdom teeth.

Though our teeth are smaller than those of Homo

erectus or Neanderthals, there is less room for them.

Negus also recognizes the problems of the modern SVT.

According to Lieberman, Negus makes it clear that, [8,11]

The right angle bend in the human vocal tract also

reduces the respiratory efficiency of our upper airways.

So we can conclude that having a human vocal tract with

a low larynx increases our chances for immediate death

by asphyxiation, increases the chances for a slower death

by infection from impacted wisdom teeth, reduces the

chances of survival when food supplies are limited (the

‘normal’ condition for most people past and present) and

restricts breathing to a degree. In fact, the only function

that is better served is speech production.

There is a cognitive piece to this story. Simply having a

modern SVT is not the only necessary component for speech

and language. Though the neural connections and develop-

ment are not described here, the brain clearly had to evolve

as well.

11. 1:1 Ratio of the SVT

A great deal of attention has been directed toward

modeling the SVT. This allows the linguist to study speech

production. Vowel formation is the most important element

of speech, and it is generally accepted, primarily from

computer models, that the maximum vocal clarity occurs

when the length of the oral cavity (SVTH) and the length of

the pharynx (SVTV) are approximately equal, i.e. the ratio of

oral cavity to pharyngeal length is approximately 1:1 [4,11].

This is shown in Fig. 6. Lieberman argues that Neanderthals

would not have been able to articulate as well as modern

man, because, given the length of the Neanderthal’s oral

cavity, the larynx would have been descended into the chest.

Homo sapiens is not a descendant of Homo neanderthalis.

Lieberman presumably uses this example to show the

importance of klinorynchy with laryngeal descent.

Fig. 5. Midsagittal view of Capra hircus (goat). Note the high position of the larynx relative to the descended position in man, the relationship of epiglottis to

soft palate (epiglottic–soft palate lockup), the facial projection, the long maxilla and mandible, the length of the sphenoid bone, the obtuse craniobase angle,

the long palate to foramen magnum distance, and the small, flat tongue, which resides exclusively in the oral cavity. From McCracken TO, Kainer RA,

Spurgeon TL. Spurgeon’s color atlas of large animal anatomy. Philadelphia: Lippincott Williams & Wilkins, 1999; p. 83. Reprinted by permission from

Lippincott Williams & Wilkins, q1999.

T.M. Davidson / Sleep Medicine 4 (2003) 185–194 191

Neanderthals have been used as the generic robust stone age

man. This is depicted in a drawing from Lieberman’s text,

and reproduced in Fig. 7. This is an important point, because

the evolution of the SVT to optimize speech required an oral

cavity and a pharynx equal in length. The descent of the

larynx and the shortening of the oral cavity accomplished

this.

The angulation between the pharynx and oral cavity

enhances the ability to produce vowel sounds [9]. While one

could argue that this angulation was part of man’s adoption

of an upright posture, it may have evolved to enhance

speech [10].

12. Evolution vs. revolution

Was there an adverse selection for sleep apnea? Other

than an occasional snorer who was killed by his cavemates,

most likely there was not negative selection for sleep apnea.

The adverse health consequences of OSA do not manifest

until the age of 40–60 years, an age well past most

reproductive activity and until recently, past the life

expectancy of Homo sapiens.

The issue of when these anatomic and behavioral

changes occurred is still an issue of great discussion

among modern anthropologists [13]. There is controversy

over whether the changes occurred rapidly, i.e. a revolution

in 40–50 ka, or more slowly, first appearing 250–300 ka

ago. The vast body of literature on this subject has other

explanation for the anatomic changes discussed herein.

Most focus on bipedalism, binocular vision and locomotion.

This paper does not side with any one theory. The

described changes may have had selective advantage for

reasons other than speech. The important point is that

speech contributed to upper respiratory tract evolution and

the changes that occurred in the SVT anatomy. These

changes, perhaps the final changes in the upper respiratory

tract, had the adverse outcome of obstructive sleep apnea.

13. Discussion

To recapitulate, modern Homo sapiens’ upper respiratory

tract anatomy evolved for several reasons. One reason was

to facilitate speech. The pharynx was narrowed to form a

narrow, distensible tube for better sound modulation by

rotation of the foramen magnum anteriorly, migration of the

palate posteriorly and shifting of the tongue into the

oropharynx. Oral/buccal speech was generated as the larynx

descended and the soft palate shortened, causing loss of the

epiglottic–soft palate lock-up. A 1:1 ratio of the SVTV to

SVTH, vocal cords to pharynx and pharynx to incisors/lips

was created by laryngeal descent and klinorynchy, the

foreshortening of the face by contraction of the ethmoid,

maxilla, palate and mandible. Craniobase angulation further

improved vocal quality.

The obstructing anatomy is clearly a soft tissue

phenomenon, as it is absent during the day and is present

at night. The soft tissue is suspended and supported by the

underlying skeleton. Soft tissue obstruction sites during

sleep include the nose, upper and lower pharynx and

occasionally the larynx. The premise of this paper is that the

changes in skeletal anatomy have positioned the soft tissues

so that they now obstruct respiration during sleep. Obesity

and old age compound sleep disordered breathing (SDB),

obstructive sleep apnea (OSA) included.

SDB is a prevalent, morbid and mortal illness [14,15]

affecting 24% of adult males and 9% of adult females [16].

SDB is a risk factor for hypertension [12–19]. It causes

early death by stroke and heart attack. It complicates all

cardiovascular disease, especially for those with angina,

heart failure, TIAs, and for CVA survivors. SDB

Fig. 6. Ratios of distances from incisor to pharynx and pharynx to larynx in Homo sapiens and Pan troglodytes. The 1:1 SVTV to SVTH ratio is shown on the

left. For comparison, the same ratio for the common chimpanzee is shown on the right. Visible Productions, 2001.

T.M. Davidson / Sleep Medicine 4 (2003) 185–194192

predisposes to accidents on the road, at home and at work.

SDB causes daytime sleepiness with loss of creative

productivity, diminished personal energy and failing

personal relationships, since snoring is associated with

bedroom disharmony. SDB is also associated with heart

failure, hypercapneic COPD, nocturia, retinal hemorrhage,

epilepsy and pacemaker dependant arrhythmia [14,15].

14. Conclusion

OSA is an adverse consequence of man’s upper

respiratory tract evolution. Speech was a substantial

contributing factor. This has important implications for

further study. Anthropologists must add this to their study of

evolutionary change. Investigation of the genetics of

craniofacial growth and development as well as the genetics

of growth and development of the SVT is essential to

current and future knowledge. Perhaps we can genetically

modify or orthodontically create a larger or wider mandible

and maxilla. Given that we do not want to impair speech, is

it possible to genetically or surgically modify the mandible,

maxilla, palate, pharynx and tongue to maintain vocal

quality, to reduce the propensity for OSA? It may be that the

oropharyngeal tongue must be studied by neurobiologists

with the goal of making this a muscle of respiration, which

contracts during sleep [20]. Perhaps there are other

contributing soft tissue changes such as a floppy epiglottis

or lax pharyngeal musculature.

Most of the anthropologic study has focused on the

lateral view, i.e. the sagittal section, and to a lesser degree,

on the base view (axial examination). Study is needed for

the axial and coronal perspectives.

Is the SVT still evolving? Can the situation worsen? Are

we raising a society condemned to nightly positive airway

pressure (CPAP) for the second half of their lives? It should

be obvious that these evolutionary changes are also the

anatomic basis for difficult laryngoscopy and intubation.

The descended larynx, the oropharyngeal tongue and the

acute craniobase angulation are the primary contributors.

Mandibular and maxillary shortening are contributing

factors for infection and dysfunction associated with

unerupted third molars. This may contribute to tempor-

omandibular joint dysfunction syndrome and orthodontic

issues as well. Foreshortening of the maxillary and ethmoid

bones may have squeezed the osteomeatal complex, folding

the uncinate over the ethmoidal infundibulum and compres-

sing the drainage space for the paranasal sinuses, thereby

predisposing a.m. Homo sapiens to chronic sinusitis.

Aspiration and acute airway obstruction are also a

consequence of the laryngeal descent.

It is interesting to speculate on the relative fitness

evolutionary pressures of bipedalism, binocular vision,

locomotion and speech. Bipedalism must have come first.

Locomotion contributed throughout. Even today, speed is

an asset. Binocular vision is a nice theory, but in reality,

does not require a shorter snout. Oral speech occurred late in

the evolutionary process, but once it appeared, exhibited

high fitness advantage and substantially influenced the final

and recent changes in the upper respiratory tract.

The views presented in this paper represent an anatomic

perspective. A separate and concurrent series of events must

have occurred in the central nervous system and contributed

to speech, sleep and obstructive sleep apnea.

Acknowledgements

Special thanks to Hilary Rogers and Barbara Stribling for

their assistance with preparing this manuscript, to Jon

Benumof, M.D. and James Moore, Ph.D. for their critical,

constructive comments, and to Lewis Sadler of Visible

Productions for his work on the illustrations. Supported by a

grant from The Farrell Fund of the San Diego Foundation.

Fig. 7. Why a Neanderthal could not have a human vocal tract. “If we place

a modern human tongue that corresponds to a mouth and pharynx that have

equal lengths on the ‘classic’ La Chapelle-aux-Saints fossil, the larynx ends

up positioned below the neck. That’s most unlikely. The tongue contour for

this sketch was derived from cineradiographic studies of living humans

talking by Peter Ladefoged and his colleagues at UCLA; it is the tongue of a

normal adult woman. A smaller tongue would not be able to push food

backward and downward to allow the Neanderthal to swallow. The spinal

column of the Neanderthal is modern and has the lordosis, or curvature, of a

normal human adult. The skull is also positioned on the standard ‘Frankfurt’

plane used to compare the skulls of various primates. The problem arises

because the length of the Neanderthal mouth is outside the range of modern

human beings. William Howells at Harvard University established this fact

by studying different human population groups.” From Lieberman [11], p.

93. Reprinted by permission from W.W. Norton & Co. Inc, q1998.

T.M. Davidson / Sleep Medicine 4 (2003) 185–194 193

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